Interstate 255 in Illinois forms the majority of the Metro-East portion of the belt-loop for the St. Louis Metro. I-255 enters Illinois at the J.B. Bridges on the southeastern side of the StL Metro and heads north and ends at the junction of Interstate 270 and Illinois State Route 255 on the north east side of the metro area.

Southbound I-255 near Mousette Lane in Centreville, IL

While in Illinois I-255 is multi-plexed several times...

I-255 is multi-plexed with...
...U.S. Rt. 50, from the Missouri/Illinois State Line to Interstate 64.